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Originally Posted by MNBoxster
You may not have a problem driving an empty Car though the Nebraska Flatlands on a 30° Day at 50MPH with the CC on, but load up the Car with Passenger, Luggage, full Fuel Tank, or drive Up the Rockies or through the Desert at 120°F, and you could create serious problems. Ambient Temperature, Altitude, and Load are major contributors to Knock. Ever notice when you hear an old Car knocking it's usually accelerating or going uphill?
Happy Motoring!... Jim'99
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I agree that temperature, altitude, and load are contributors to knock. But that old car knocking is generally because that vehicle doesn't have the modern computers capable of adjusting the spark timing and air-fuel ratio.
In truth, an increase in altitude would decrease the necessary (R+M)/2 number by 1-2 per 3000' elevation. As such, octane numbers for gas in the Rocky Mountain region are lower than those in lower altitude areas (87 is considered "mid-grade" with 89 to 91 as the "premium" here). As such, wouldn't you actually be better off increasing in altitude with 87?